Ball valves basically consist of a body comprising a longitudinal duct for the fluid flow, in the center of which a ball is tightly interposed; having said ball a cylindrical perforation which is axial to the fluid flow when the valve is in a fully open condition. Said ball is joined to an actuating stem which is perpendicular to both the duct and the cylindrical perforation. Thus, the actuating stem allows the rotation of the ball from an obstructing position with its perforation perpendicular to the duct to its fully open condition, by means of a 90-degree turn, with its perforation coaxial to the duct.
Due to the fact that the ball is sustained between two ring-shaped seats which, traditionally, perform as ball supports as well as sealing elements, this type of valve presents inconveniences restricting its fluid-tightness efficiency when subjected to temperature changes (although to a lesser degree than in other types of valves), because of the different thermal expansion coefficients possessed by the valve bodies, the seats, and the ball. This traditional design aspect also restricts the usage lifetime of the valve when it is used in positions intermediate to the fully open and closed conditions, because the ring-shaped seats become partially worn out and, since said seats also act a sealing elements, the fluid-tightness of the valve becomes impaired.